The objectives of this research are to investigate (1) effects of ionizing radiation on the immature spinal cord, (2) the reparative and regenerative capacities of the immature nervous system, and (3) the manner in which ionizing radiation alters these capacities. The animal used for this study is the laboratory rat because its developmental state at birth permits the study of immature spinal cord ex utero. The experiments will deal with the effects of direct (surgical intervention or stab wound) and indirect (sciatic axotomy) injury on both irradiated and non-irradiated spinal cord. The patterns of normal development and their alterations are studied by histopathologic (light microscopy) and autoradiographic methods. Responses of the immature system will be compared with those of the mature system in an attempt to gain some insight into the responses and their limitations occurring in the mature state.